Carbonation of Concrete

The rate of carbonation depends on concrete permeability, then it decreasing with low w/c ratio, and with cement content increase. Therefore, the rate of caibon-ation is inversely proportional to the strength of concrete. 

The well crystallized, autoclaved calcium silicate hydrates show low caibon-ation rate in spite of the higher lime content, than the amorphous forms of tober-morite, poorer in calcium [298]. However, Roy [80] found that the C-S-H richer in calcium than tobermorite, was readily carbonated and the eqnilibrinm CO2 partial pressure was lower. 

The following properties are altered as a consequence of concrete carbonation shrinkage (see Sect. 5.3.2), strength, porosity, susceptibility to deformations, and resistance to the environmental impact. However, the pH of pore solution in concrete decreases and the passive film on steel is deteriorated therefore, the reinforcement is exposed to corrosion. This corrosion is probably the most frequent reason of concrete deterioration, because the mst formation causes the surrotmding concrete to crack and spall (see Sect. 6.4.11.). 

The pores stmcture is affected by carbonation too. The total porosity decreases and the pores size distribution is shifted towards larger dimensions. The larger pores are not specially altered, but the volume of smaller became lower. The diffusion rate in carbonated concrete becomes low, as well as the permeability, particularly at low w/c ratio. On the other hand the increased porosity was found in the case of slag 

Analyzing these data it should be remember that the carbonation rate depends on the permeability of concrete, it means that is decreasing with lowering of w/c ratio 

In moist environments, carbon dioxide present in the air forms an add aqueous solution that can react with the hydrated cement paste and tends to neutralize the alkalinity of concrete (this process is known as carbonation). Also other acid gases present in the atmosphere, such as SO2, can neutralize the concrete s alkalinity, but their effect is normally limited to the surface of concrete. 

The alkaline constituents of concrete are present in the pore liquid (mainly as sodium and potassium hydroxides. Section 2.1.1) but also in the solid hydration products, e. g. Ca(OH)2 or C-S-H. Calcium hydroxide is the hydrate in the cement paste that reacts most readily with CO2. The reaction, that takes place in aqueous solution, can be written schematically as  

This is the reaction of main interest, espedally for concrete made of Portland cement, even though the carbonation of C-S-H is also possible when Ca(OH)2 becomes depleted, for instance by pozzolanic reaction in concrete made of blended cement [1]. 

Carbonation does not cause any damage to the concrete itself, although it may cause the concrete to shrink. Indeed, in the case of concrete obtained with Portland cement, it may even reduce the porosity and lead to an increased strength. However, carbonation has important effects on corrosion of embedded steel. The first consequence is that the pH of the pore solution drops from its normal values of pH 13 to 14, to values approaching neutrahty. If chlorides are not present in concrete initially, the pore solution following carbonation is composed of almost pure water (Section 2.1.1). This means that the steel in humid carbonated concrete corrodes as if it was in contact with water [2, 3]. A second consequence of carbonation is that chlorides bound in the form of calcium chloroaluminate hydrates and otherwise bound to hydrated phases may be Hberated, making the pore solution even more aggressive [2-4]. 

Luca Bertolini, Bernhard Elsener, Pietro Pedeferri, Rob P. Polder Copyright 2004 WILEY-VCH Verlag GmbH Co. KGaA, Weinheim ISBN 3-527-30800-8 

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R49 Raask, E., in International Symposium on the Carbonation of Concrete (Proc. 

Fukushima, T. (1991) Predictive Methods on the Progress of Neutralization (Carbonation) of Concrete by Unsteady State Dynamic Analysis Considering the Influence of Tendency of Increase in Concentration of Atmospheric Carbon Dioxide, Proceedings of the 2nd CANMET/ACI International Conference, held in Montreal, Canada, August 4—9, 1991, edited by V.M.Malhortra, Supplementary Papers, American Concrete Institute, pp.-545-564. 

From the viewpoint of prediction of service lives, the photochemical deterioration processes of polymers used as paints and finishes are theoretically analyzed based upon unsteady state dynamics. Theoretical results are compared with experimental data under natural and accelerated exposure. Infrared spectra and scanning micrographs show that the deterioration proceeds continuously inwards from the surface, but differently with the exposure conditions. Parabolic (/t ) law was derived approximately for the increase in the depth of the deteriorated layer of polymers with time. Paying attention to the influence of the deterioration of polymeric finishes, the parabolic law involving a constant term was also derived for the progress of carbonation of concrete. These parabolic laws well predict the progress of deterioration and explain the protective function of finishes on reinforced concrete. 

This report deals with dynamic processes of the deterioration of polymers often used as paints and finishes in housing, and also refers to their influence as the reduction in protective performance on the durability of reinforced concrete. The deterioration processes of polymers by the simiiltaneous action of ultraviolet (UV) light and diffusive oxygen is explained theoretically based upon unsteady state dynamics. The parabolic law (/t" law) is derived for a typical path for the progress of the deterioration of polymers inwards from the surface (l), and compared with some experimental data. The same parabolic law involving a constant term was also derived for the carbonation of concrete, which well explains the retardation effects of finishes on the carbonation (2). 

Paying attention to the influence of the deterioration of polymeric finishes, the parabolic law involving a constant term was also derived for the progress of carbonation of concrete. These parabolic laws combined predict the progress of the deterioration under natural weathering and explain well the protective performance of finishing materials on reinforced concrete. 

During hydration of cement a highly alkaline pore solution (pH between 13 and 13.8), principally of sodium and potassium hydroxides, is obtained (Section 2.1.1). In this environment the thermodynamically stable compounds of iron are iron oxides and oxyhydroxides. Thus, on ordinary reinforcing steel embedded in alkaline concrete a thin protective oxide film (the passive film) is formed spontaneously [1-3]. This passive film is only a few nanometres thick and is composed of more or less hydrated iron oxides with varying degree of Fe and Fe [4j. The protective action of the passive film is immune to mechanical damage of the steel surface. It can, however, be destroyed by carbonation of concrete or by the presence of chloride ions, the reinforcing steel is then depassivated [5j. 

Carbonation of concrete leads to complete dissolution of the protective layer. Chlorides, instead cause localized breakdown, unless they are present in very... 

Figure 5.1 Schematic representation of the rate of carbonation of concrete as a function of the reiative humidity of the environment, under equiiibrium conditions [4]...

Carbonation of concrete can be modelled relatively simple with good accuracy (Chapter 5). The following model is a simplified representation of the DuraCrete model for carbonation-induced corrosion initiation [21], Initiation of corrosion by carbonation may be set as a hmit state. The design equation g is then given by... 

Once the steel becomes active due to carbonation of concrete or chloride penetration, the influence of chemical composition, microstructure and surface finish-... 

Carbonation of concrete is associated with loss of alkalinity of the pore solution. This change in pH can be revealed by a suitable indicator that changes colour near pH 10. A phenolphthalein solution will remain colourless where concrete is carbonated and will turn pink where concrete is still alkaline. The best indicator solution for maximum contrast of the pink coloration is a solution of phenolphthalein indicator in alcohol and water, usually 1 g of indicator in 100 ml of 50 50 or more alcohol to water mix. 

BRE Digest 405 (1995). Carbonation of Concrete and its Effects on Durability. Building Research Establishment, Garston, UK, Publ CRC Ltd. London. 

The basic aggression can be, in principle, due to the action of sodium and potassium carbonates, particularly to the sodium compounds [67], This can be considered as promoted by sodium lye together with carbonation of concrete. In this condition the sodium carbonate hydrates are formed and concrete is damaged by their crystallization. 

The corrosion of reinforcement is discussed in details in a book by Gizegorz Wieczorek, Concrete corrosion initiated by ehlorides or carbonation of concrete cover , Editor Dolno skie Wydawnictwo Edukacyjne, Wroclaw 2002. (in Polish). 

Carbonation of concrete is the result of carbon dioxide gas diffusing into the concrete pores. Carbonation rates tend to be highest when the concrete is relatively dry since the pores contain little water to prevent entry of the gas but just sufficient to allow it to dissolve. Carbonic acid is formed which reacts with the free lime in concrete to form calcium carbonate and leads to a gradual fall in alkalinity from the surface inwards. Once the carbonation fi-ont reaches the steel reinforcement depassivation occurs and, in the presence of water and oxygen, corrosion can proceed. 

Nakamoto, J., and Togawa, K. (1995) A study of strength development and carbonation of concrete incorporating big volume blastfurnace slag. American Concrete Institute SP-153,pp. 1121-1139. 

Hobbs, D.W. (1988) Carbonation of concrete containing PFA. Magazine of Concrete Research 40 (143), 69-78. 

Building Research Establishment (1981) Information Paper IP6/81 Carbonation of Concrete made uhth Dense Natural Aggregates, building Research Establishment, Garshm, Watford. 

The pH of the electrolyte does not only have an effect on the passivation potential, but also on the passivation current density, because both the metal dissolution kinetics and the solubility of hydroxides depend on pH. Figure 6.16 shows that the passivation current density of iron becomes smaller at higher pH. This has been explained by a lowering of the solubility of ferrous hydroxide, which precipitates at the surface. Since both the passivation potential and the passivation current density decrease with increasing pH, spontaneous passivation of iron becomes possible in basic, aerated media. This explains why steel reinforcements in concrete (pH >13) resist corrosion well as long as chemical reactions with carbon dioxide from air (carbonation of concrete) do not modify the alkalinity. 

Though not considered a major problem in North America, carbonation of concrete is a major cause of steel corrosion in the world. Calcium hydroxide in the concrete can react with carbon dioxide or carbon monoxide to produce calcium carbonate. The calcium carbonate is not detrimental to the mechanical properties, but in the process the pH of the concrete drops below 10. At that point the corrosion rate of embedded steel can significanfly increase. This phenomenon is mostly found in concretes with low cement contents, high permeabiUly, and low concrete cover over the rebars. 

If the first effect can be considered beneficial for the durability of the matrix itself, the second one increases the danger of accelerated corrosion of steel-reinforcement, which is by far the most important reason for deterioration of reinforced concrete structures exposed to open air. The steel is prevented from corrosion as long as the passive oxide film is maintained, but the carbonation of concrete destroys that film. The depth and density of the cover concrete over the steel reinforcement decides on the structure s durability. 

It is difficult to determine a relationship between the results of accelerated tests and of the natural exposure of certain durations. However, many attempts are published in which a safe life cycle is estimated from the results of accelerated tests. For example, it has been proposed that exposure of concrete elements to pure CO2 over a period of 36 days with a ratio of concentration 3000 times higher may give the same carbonation of concrete as 300 years of service in natural atmosphere (Levy 1992). In the tests carried out by Sisomphon and Franke (2007) it has been derived on the basis of the second Fick s law that the carbonation in natural exposure with concentration CO2 of 0.03% is approximately 10 times slower than in the accelerated tests at concentration CO2 of 3%. 

These processes, accelerated by the carbonization of concrete, cause the formation of active-passive cells on the surface of steel, intensifying corrosion of the reinforcement, It is considered that chloride contents in concrete should not exceed 0.4% by weight in relation to the dry mass of cement. In many stated cases of corrosion of reinforced concrete structures, a higher chloride content was observed. 

Carbonization of concrete caused by the penetration of carbon dioxide into concrete leads initially to the formation of calcium carbonate as a result of reaction with calcium hydroxide, and then to calcium hydrogen carbonate, which is washed out by rain water from concrete capillaries. A decrease of the pH of the concrete environment to 8-8.5 is the result and it causes destruction... 

Curiously, one of the recent proposals for protective treatment for surface protection has been to promote the formation of calcium oxalate coatings (Doherty et al. 2007 Pinna et al. 2011) that has been assessed on the field and can also apparently have some consolidating effect (Doherty et al. 2007). One can also consider here, in a similar vein, treatments used to stop carbonation of concrete such as realkaliniza-tion (Yeih and Chang 2005 Redaelli and BertoUni 2011). 

For a number of technical calculations, knowledge of the properties of gases is necessary. For example, this applies to treatment of subjects such as hygroscopic moisture absorption, carbonation of concrete, gas permeability of porous materials, and moisture diffusion in materials. For technical calculations, ideal gases are often assumed, which in most cases is an acceptable assumption. For these kinds of calculations, the concept of ideal gas is briefly explained in the following portions of this section. 

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